A hydraulic brake system with a primary circuit and a secondary circuit is described in German patent application No. 43 37 133. The term `primary circuit` refers to the connecting lines and channels which connect the master brake cylinder of the brake system to the wheel brakes and through which brake fluid is conducted in the direction of the wheel brakes during normal braking operations. The term `secondary circuit` refers to those connecting lines and channels which are penetrated by pressure media in the case of wheel slip control.
The secondary circuit is connected to the primary circuit by way of an outlet valve on the inlet side and by way of a so-called return pump on the outlet side. The brake system is filled with a conventional brake fluid as a pressure medium. Filling is effected at the end of the vehicle assembly on the assembly line, that means, after the brake system has been installed into the vehicle in dry condition. The following procedure has proved to be quick and unproblematic to carry out. Initially, the entire system is evacuated and, in the evacuated condition, is thereafter connected to a brake fluid reservoir. The result is that brake fluid enters into the brake system and first fills the primary circuit. In order that the secondary circuit is also filled with pressure fluid, the electromagnetically operable outlet valve is opened during this process, i.e., both during evacuation and filling. This is disadvantageous inasmuch as provision must be made at the filling station to switch the normally closed outlet valve to adopt its open position.
Therefore, it has been suggested in German patent application No. 43 37 133 to interpose a non-return valve, which opens towards the primary circuit, between the secondary circuit and the primary circuit. This valve will open during evacuation of the system so that both circuits are evacuated. When the valve closure member is not acted upon by a spring in the closing direction, as is disclosed in the first embodiment of the above publication, the valve remains open at the end of the evacuation process so that both the primary circuit and the secondary circuit are filled with pressure fluid during the filling process. In a second embodiment, it is emphasized as favorable that the non-return valve closes after the evacuation process. According to the application, this is said to be achieved because the valve closure member is arranged above the valve seat and urged against the valve seat by the force of gravity. In a wheel slip control operation which is induced on a roller test bench, where the outlet valve is opened and the return pump switched on, brake fluid flows into the secondary circuit.
The disclosure of German patent application No. 43 37 133 ensures a rapid and simple method of filling the brake system with brake fluid during the vehicle assembly. However, the additional installation of a non-return valve between the secondary circuit and the primary circuit involves some shortcomings. One disadvantage is the need for absolute seal-tightness of the valve because otherwise master cylinder pressure is built up in the secondary circuit during a braking operation, with the result that pressure reduction is not possible in the wheel brakes during wheel slip control.
A metal/metal seal pairing for the valve seat and the valve closure member is inappropriate because contaminants or chips may gather between the valve seat and the valve closure member and prevent complete closure of the valve.
Therefore, generic German patent application No. 19632343 disclosed making at least the sealing lip of the valve closure member of a relatively soft plastic material which receives and stores the contaminants contained in the brake fluid so that reliable closure of the non-return valve is ensured. Further, it has been disclosed to load the valve closure member with a weak spring so that both circuits are evacuated during the evacuation process, similar to the second suggestion made in German patent application No. 43 37 133. Initially, only the primary circuit is filled with brake fluid during the filling process, while the secondary circuit is filled by opening the outlet valve in an induced wheel slip control operation. However, this solution, too, involves problems, especially in those cases where the secondary circuit is constituted by a channel in a so-called valve block made of steel or aluminum. Upon brake application, where pressure develops in the primary circuit, the pressure is applied to the valve closure member which, by compression of its elastically yielding sealing lip, is displaced in the direction of the secondary circuit. The volume decreases which is provided for the pressure fluid in the secondary circuit, with the result that a corresponding part of the brake fluid is displaced past the sealing lip into the primary circuit. After release of the brakes, that is, after the pressure in the primary circuit has been reduced again, the valve closure member is shifted back again, under the elastic effect of its sealing lip. The space in the secondary circuit which is available for the pressure fluid will re-increase, with the result that a vacuum develops therein which is not desirable. One possibility of overcoming the problem would be to increase the bias of the valve spring, which, in turn, would prevent a complete evacuation of the secondary circuit. This would necessitate a special filling process.
Thus, an object of the present invention is to configure the non-return valve so that it is appropriate to eliminate the above-mentioned problems when used in the automotive vehicle brake system with wheel slip control described hereinabove.
To this end, the present invention suggests the provision of a blocking device which cannot be released, at least in the normal operation of the brake system, and which, in the blocking condition, is appropriate to keep the valve closure member in the closing position.
Those conditions shall be understood as the `normal operation` of a brake system, where pressure develops in the primary circuit. The purpose of the blocking device is to prevent a pressure fluid exchange by way of the non-return valve.
The blocking device is so configured that it becomes effective when the valve closure member is loaded by pressure for the first time, i.e., during a braking operation.
For example, the blocking device can be a unilaterally acting blocking device, i.e., it becomes engaged as soon as the valve closure member has been displaced in the direction of the secondary circuit.
The blocking device includes a lock bar which is resiliently yieldingly arranged on the valve closure member and engages a (main) catch on the valve seat member.
Also, the lock bar can be utilized to retain the valve closure member on a first catch on the valve seat member to permit installation of the unit made up of valve spring, valve closure member, and a valve seat member into a corresponding housing.
Preferably, the catches are provided on a valve seat member in which the valve passage is contained on the side remote from the valve closure member, and the lock bar is arranged on an elastically yielding holder element which extends through the valve passage.